Digital systems commonly operate in noisy conditions. Noisy conditions are usually defined as environments where there are signals unrelated to the signal of interest. In some cases, the noise content can be so large that false switching occurs. That is, noise can cause a digital low signal to appear as a digital high signal or vice versa.
On occasion, noise conditions cause erroneous signal glitches between circuit nodes that should otherwise be isolated. This problem is especially common in those cases where the two nodes are at times connected and at times disconnected by an isolation circuit, such as a multiplexer or pass transistor. For example, assume there is an input node that is supposed to be isolated from an output node with an n-channel pass transistor whose gate is grounded. The pass transistor will turn-on if the undershooting signal on the input node is more negative in magnitude than the Vtn of the pass transistor. Conversely, for a p-channel pass transistor, if the overshoot signal is greater than the Vtp of the pass transistor, it too will turn-on. In both cases, a signal glitch occurs on the output node that was supposed to be isolated.
In view of the foregoing, it would be highly desirable to provide an isolation circuit that prevents signal glitches in the presence of undershoot and overshoot signals. Ideally, such a circuit should be simple to implement and should otherwise be compatible with existing circuit designs.